6 Morgan, Ste156, Irvine CA 92618 · P: 949.461.9292 · F: 949.461.9232 · nanovea.com Today's standard for tomorrow's materials. © 2014 NANOVEA

INDUSTRIAL COATING SCRATCH AND WEAR EVALUATION

Prepared by Duanjie Li, PhD & Andrea Herrmann

2

INTRO Acrylic urethane paint is a type of fast-dry protective coating widely used in a variety of industrial applications, such as floor paint, auto paint, and others. When used as floor paint, it can serve areas with heavy foot and rubber-wheel traffic, such as walkway, curbs and parking lots. IMPORTANCE OF SCRATCH AND WEAR TESTING FOR QUALITY CONTROL Traditionally, Taber abrasion tests were carried out to evaluate the wear resistance of the acrylic urethane floor paint according to ASTM D4060 Standard. However, as mentioned in the ASTM D4060 standard, “For some materials, abrasion tests utilizing the Taber Abraser may be subject to variation due to changes in the abrasive characteristics of the wheel during testing.” [1] This may result in poor reproducibility of test results and create difficulty in comparing values reported from different laboratories. Moreover, in Taber abrasion test, abrasion resistance is calculated as loss in weight at a specified number of abrasion cycles. However, acrylic urethane floor paints have a recommended dry film thickness of 37.5-50 μm [2]. The aggressive abrasion process by Taber Abraser can quickly wear through the acrylic urethane coating and create mass loss to the substrate, which leads to substantial errors in the calculation of the paint weight loss. The implant of abrasive particles in the paint during the abrasion test also contributes to the errors. Therefore, a well-controlled quantifiable and reliable measurement is crucial to ensure reproducible wear evaluation of the paint. In addition, the scratch test allows users to detect premature adhesive/cohesive failure of the paint/substrate system in real-life applications. MEASUREMENT OBJECTIVE The wear process of the acrylic urethane floor paints with different topcoats is simulated in a controlled and monitored manner using the Nanovea Tribometer as shown in Fig. 1. Micro scratch testing is used to measure the load required to cause cohesive or adhesive failure to the paint. In this study, we would like to showcase that Nanovea Mechanical Tester and Tribometer are ideal tools for evaluation and quality control of commercial floor and automotive coatings.

Fig. 1: pin-on-disk wear test.

3

TEST PROCEDURE

SAMPLE DISCRIPTION

The coatings for this study are four commercially available water based acrylic floor coatings that have the same primer (basecoat) and different topcoats of the same formula with a small alternation in the additive blends for the purpose of enhancing durability. These four coatings are identified as Samples A, B, C and D in this study. WEAR TEST Nanovea Tribometer, as illustrated in Fig. 2, was applied to evaluate the tribological behavior, e.g. coefficient of friction, COF, and wear resistance. A SS440 ball tip (6 mm dia., Grade 100) was applied against the tested paints. The COF was recorded in situ. The test parameters are summarized in Table 1. The wear rate, K, was evaluated using the formula K=V/(F×s)=A/(F×n), where V is the worn volume, F is the normal load, s is the sliding distance, A is the cross-sectional area of the wear track, and n is the number of revolution. Surface roughness and wear track profiles were evaluated by the Nanovea Optical Profilometer, and the wear track morphology was examined using optical microscope.

Fig. 2: Schematic of the pin-on-disk test.

Test parameters Value Normal force 20 N Speed 15 m/min Duration of test 100, 150, 300 and 800 cycles Rotational speed Adjusted according to change of radii

Table 1: Test parameters of the pin-on-disk measurement.

Adjustable weights (Load)

Sample stage

Pin or ball holder

Wear track

Tribometer arm

Strain gauge (for lateral force measurement)

To arm pivot and displacement motor (wear track radius adjustment)

4

SCRATCH TEST Nanovea Mechanical Tester (P-Micro/Nano P-10-0123) equipped with a Rockwell C diamond stylus (200 μm radius) was used to perform progressive load scratch tests on the paint samples using Micro Scratch Tester Mode as shown in Fig. 3. Two final loads were used: 5 N final load for investigating paint delamination from the primer, and 35 N for investigating primer delamination from the metal substrates. Three tests were repeated at the same testing conditions on each sample to ensure reproducibility of the results.

Fig. 3: Schematic of scratch testing. Images of the whole scratch (panorama) were automatically generated and the different areas of the scratch were correlated with the applied loads by the system software. This facilitates users to perform analysis on the scratch tracks any time, rather than having to determine the critical load under the microscope immediately after the scratch tests. The test parameters are listed in Table 2.

Test parameters Value Load type Progressive Initial Load 0.01 mN Final Load 5 N / 35 N Loading rate 10 / 70 N/min Scratch Length 3mm Scratching speed, dx/dt 6.0 mm/min Indenter geometry 120° cone Indenter material (tip) Diamond Indenter tip radius 200 μm

Table 2: Scratch test parameters.

5

RESULTS AND DISCUSSION WEAR TEST RESULTS Four pin-on-disk wear tests at different number of revolutions (100, 150, 300 and 800 cycles) were performed on each sample in order to monitor the evolution of wear process. The samples show similar surface morphology and comparable roughness of ~1 μm as displayed in Fig. 4. The COF was recorded in situ during the wear tests as shown in Fig. 5. Fig. 6 presents the evolution of wear tracks after 100, 150, 300 and 800 cycles, and Fig. 7 summarized the average wear rate of different samples at different stages of the wear process. Compared with a COF value of ~0.07 for the other three samples, Sample A exhibits a much higher COF of ~0.15 at the beginning, which gradually increases and gets stable at ~0.3 after 300 wear cycles. Such a high COF accelerates the wear process and creates a substantial amount of paint debris as indicated in Fig. 6 – the topcoat of Sample A has started to be removed in the first 100 revolutions. As shown in Fig. 7, Sample A exhibits the highest wear rate of ~5 μm2/N in the first 300 cycles, which slightly decreases to ~3.5 μm2/N due to the better wear resistance of the metal substrate. The topcoat of Sample C starts to fail after 150 wear cycles as shown in Fig. 6, which is also indicated by the increase of COF in Fig. 5. In comparison, Samples B and D show enhanced tribological properties. Sample B maintains a low COF throughout the whole test – the COF slightly increases from~0.05 to ~0.1. Such a lubricating effect substantially enhances its wear resistance – the topcoat still provides superior protection to the primer underneath after 800 wear cycles. The lowest average wear rate of only ~0.77 μm2/N is measured for Sample B at 800 cycles. The topcoat of Sample D starts to delaminate after 375 cycles, as reflected by the abrupt increase of COF in Fig. 5. The average wear rate of Sample D is ~1.1 μm2/N at 800 cycles. Compared to the conventional Taber abrasion measurements, Nanovea Tribometer provides well-controlled quantifiable and reliable wear assessment that can ensure reproducible evaluation and quality control of the commercial floor/auto paints. Moreover, the capacity of in-situ COF measurement allows users to correlate different stages of wear process with the evolution of COF, which is critical in improving fundamental understanding of the wear mechanism and tribological characteristics of various paint coatings.

6

(a) Sample A: R = 1.14 μm

(b) Sample B: R = 1.08 μm

(c) Sample C: R = 0.93 μm (d) Sample D: R = 1.05 μm

Fig. 4: 3D morphology and roughness of the paint samples.

0 200 400 600 800 10000.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

CO

F

Revolution

Sample A

Sample C

Sample D

Sample B

Fig. 5: Coefficient of friction during pin-on-disk tests.

7

100 cycles 150 cycles 300 cycles 800 cycles (a) A:

(b) B:

(c) C:

(d) D:

Fig. 6: Evolution of wear tracks during the pin-on-disk tests.

150 300 800

0

1

2

3

4

5

6

7

8

Wea

r ra

te (

um

2/N

)

Num ber of revolutions

Sam ple A Sam ple B Sam ple C Sam ple D

Fig. 7: Evolution of wear rate of different paints.

8

SCRATCH TEST RESULTS Fig. 8 shows the plot of normal force, frictional force and true depth as a function of scratch length for Sample A as an example. An optional acoustic emission module can be installed to provide more information. As the normal load linearly increases, the indention tip gradually sinks into the tested sample as reflected by the progressive increase of true depth. The variation in the slopes of frictional force and true depth curves can be used as one of the implications that coating failures start to occur.

Fig. 8: Normal force, frictional force and true depth as a function of scratch length for scratch test of Sample A with a maximum load of 5 N.

Fig. 9 and Fig. 10 shows the full scratches of all four paint samples tested with a maximum load of 5 N and 35 N, respectively. Sample D required a higher load of 50 N to delaminate the primer. Scratch tests at 5 N final load (Fig. 9) evaluate the cohesive/adhesive failure of the top paint, while the ones at 35 N (Fig. 10) assess the delamination of the primer. The arrows in the micrographs indicate the point at which the top coating or the primer starts to be completely removed from the primer or the substrate. The load at this point, so called Critical Load, Lc, is used to compare the cohesive or adhesive properties of the paint as summarized in Table 3. It is evident that the paint Sample D has the best interfacial adhesion – exhibiting the highest Lc values of 4.04 N at paint delamination and 36.61 N at primer delamination. Sample B shows the second best scratch resistance. From the scratch analysis, we show that optimization of the paint formula is critical to the mechanical behaviors, or more specifically, scratch resistance and adhesion property of the acrylic floor paints.

A

9

Fig. 9: Micrographs of full scratch with 5 N maximum load.

(a) Sample A:

(b) Sample B:

(c) Sample C:

(d) Sample D:

10

(a) Sample A:

(b) Sample B:

(c) Sample C:

(d) Sample D (50 N):

Fig. 10: Micrographs of full scratch with 35 N maximum load.

Paint Delamination (N) Primer Delamination (N) Sample

Test 1 Test 2 Test 3 Average Test 1 Test 2 Test 3 Average A 3.01 3.21 3.27 3.16 ± 0.14 25.55 25.44 25.21 25.40 ± 0.17 B 3.45 3.36 3.53 3.45 ± 0.08 27.58 27.80 28.62 28.00 ± 0.55 C 1.61 1.09 1.57 1.42 ± 0.29 22.91 24.02 24.25 23.73 ± 0.72 D 3.94 4.02 4.17 4.04 ± 0.12 37.98 36.08 35.77 36.61 ± 1.20

Table 3: Summary of critical loads.

CONCLUSION Compared to the conventional Taber abrasion measurements, Nanovea Mechanical Tester and Tribometer are superior tools for evaluation and quality control of commercial floor and automotive coatings. Scratch testing can detect adhesion/cohesion problems in the coating system. Nanovea Tribometer provides well-controlled quantifiable and repeatable tribological analysis on wear resistance and coefficient of friction of the paints. Based on the comprehensive tribological and mechanical analyses on the water based acrylic floor coatings tested in this study, we show that Sample B possesses the lowest COF and wear rate and the second best scratch resistance, while Sample D exhibits the best scratch resistance and second best wear resistance. This assessment allows us to evaluate and select the best candidate targeting the needs in different application environments. The Nano, Micro or Macro modules of the Nanovea Mechanical Tester all include ISO and ASTM compliant indentation, scratch and wear tester modes, providing the widest range of testing available for paint evaluation on a single module. Nanovea Tribometer offers precise and repeatable wear and friction testing using ISO and ASTM compliant rotative and linear modes, with optional high temperature wear, lubrication and tribo-corrosion modules available in one pre-integrated system. Nanovea's unmatched range is an ideal solution for determining the full range of mechanical/tribological properties of thin or thick, soft or hard coatings, films and substrates, including hardness, Young’s modulus, fracture toughness, adhesion, wear resistance and many others. Optional 3D non-contact profiler is available for high resolution 3D imaging of scratch and wear track in addition to other surface measurements such as roughness. Learn More about the Nanovea Tribometer and Nanovea Mechanical Tester or Lab Services

REFERENCES

[1] ASTM D4060 - 10: Standard Test Method for Abrasion Resistance of Organic Coatings by the Taber Abraser. [2]http://www.rustoleum.com/~/media/DigitalEncyclopedia/Documents/RustoleumUSA/TDS/English/IBG/Concrete%20Saver/5600_System_CS23_CS1635.ashx.